Recently, next generation mobile communication systems have been extensively studied. As a method of enhancing the system frequency utilization efficiency, a single frequency reuse cellular system, in which each cell uses the same frequency band, has been proposed.
OFDMA (Orthogonal Frequency Division Multiple Access) is dominant in a downlink (communication from a base station device to a mobile terminal device). OFDMA is a communication system in which information data are modulated using modulation schemes, such as 64QAM (64-ary Quadrature Amplitude Modulation) or BPSK (Binary Phase Shift Keying), to generate OFDM signals, and resource blocks, each of which is an access unit defined by time-and-frequency axes, are shared by multiple mobile station devices. Since OFDM signals are used, a PAPR (Peak to Average Power Ratio) becomes very high in some cases. The high peak power is not a significant problem for downlink communication achieving a relatively-high transmission-power amplifying performance. However, the high peak power is a fatal problem for uplink communication (from the mobile station to the base station device) achieving a relatively-low transmission-power amplifying performance.
For this reason, single carrier communication schemes, in which a PAPR is relatively low, have been proposed. DFT-s-OFDM (Discrete Fourier Transform-spread-OFDM) is one of the single carrier communication schemes (see, for example, Non-Patent Document 1). FIG. 7 is a schematic block diagram illustrating a configuration of a transmission device using DFT-s-OFDM. An encoder 100 performs error correction coding on transmission data that is received information data. A modulator 101 performs modulation, such as BPSK (Binary Phase Shift Keying) (hereinafter, performing a modulation is called “first modulation” in this description). An S/P (Serial/Parallel) converter 102 converts time-domain signals into parallel signals. A DFT (Discrete Fourier Transform) unit 103 performs Fourier transform to convert the time domain signals into frequency domain signals.
The converted frequency domain signals are allocated to inputs of the IDFT (Inverse Discrete Fourier Transform) unit 105 through the subcarrier allocating unit 104 based on a rule as explained later. The IDFT unit 105 inserts 0 to IDFT points receiving no inputs, and performs IDFT to obtain a time domain waveform. A GI (Guard Interval) inserter 106 inserts a guard interval into the time domain waveform. A P/S (Parallel/Serial) converter 107 converts the time domain waveform into serial signals. A D/A (Digital/Analog) converter 108 converts the serial signals into analog signals. An RF (Radio Frequency) unit 109 upconverts the analog signals into radio frequency signals, and transmits the radio frequency signals through an antenna (not shown). For a system multiplying multiple data pieces of multiple users, the number of IDFT points is set to be greater than that of DFT points, and subcarriers to which 0 is inserted are used by other mobile terminal devices.
The data generated in this manner has a low PAPR similarly to the case of single carrier modulation. Additionally, the insertion of the guard interval enables the generated data to be processed without inter-symbol interference, similarly to OFDM signals (an interval at which a guard interval is inserted, i.e., data processing unit by which DFT is performed is called a “symbol” in this description). The frequency-domain waveform is generated by DFT, enabling easy control in the frequency domain.
Two methods have been proposed as the frequency allocation rule. One method is called “L (Localized) allocation,” and the other method is called “D (Distributed) allocation.” As shown in FIG. 8A, the L allocation is an allocation scheme in which frequency data pieces having been subjected to DFT are sequentially allocated to the inputs of the IDFT unit without changing the arrangement of the frequency data pieces. As shown in FIG. 8B, the D allocation is an allocation scheme in which the frequency data pieces are distanced at a predetermined interval and allocated to the inputs of the IDFT unit.
The L allocation can achieve the diversity effect obtained by each user selecting a suitable frequency band, i.e., the user diversity effect. The D allocation can achieve the frequency diversity effect since frequency bands are widely used. However, both allocation schemes cannot achieve selection of optimal subcarriers for communication. Sufficient performance cannot be achieved especially in a channel environment causing high frequency selectivity or in an environment affected by many interference signals from other cells.
As AMCS (Adaptive Modulation and Coding Scheme), it has been proposed that the first modulation scheme be changed based on a channel condition. This is a method in which a modulation scheme, an encoding rate, or the like is changed based on SNR (Signal to Noise Ratio) indicative of channel quality of a band to be used or SINR (Signal to Noise and Interference Ratio), and a target error rate.
On the other hand, as an uplink communication scheme, a single carrier CI (Carrier Interferometry) has been proposed (see, Non-Patent Document 2). Also in this scheme, transmission signals can be generated by the same signal generation scheme as DFT-s-OFDM. Non-Patent Document 2 has proposed flexible allocation rules with respect to the aforementioned allocation rules (L allocation and D allocation).
This is a scheme in which frequency domain signals output from the DFT unit are grouped into blocks (hereinafter, “segments”) each including several subcarriers. Then, when those signals are allocated to the inputs of the IDFT unit, subcarriers less affected by other cells are selected, and the signals are allocated to the selected subcarriers (which is, hereinafter, defined as “LS-x allocation” where x is the number of frequency domain signals allocated to the same segment). This allocation can enable selection of subcarriers achieving higher communication precision, compared to the aforementioned L allocation. If only the optimal allocation is considered, the PAPR characteristics degrade. If the number of frequency domain signals included in each segment is increased, however, the degradation of the PAPR characteristics can be reduced. If the number of frequency domain signals included in each segment is 1, optimal subcarriers can be selected. In this case, a PAPR increases and approximates that of the OFDM signals allocated to the same number of subcarriers.
The following fact is known (see Non-Patent Document 2). As the number of frequency domain signals included in each segment increases, the PAPR decreases, i.e., the PAPR characteristics become better. The better PAPR characteristics indicate a low probability of a high instantaneous power. As the number of frequency domain signals included in each segment decreases, the error rate characteristics become better. Hereinafter, in this description, LS-xx denotes a scheme in which the number of frequency domain signals included in each segment is xx.
Additionally, it is known as an adaptive control that if the number of frequency domain signals to be used is set to be half or quarter the usual number (i.e., half rate or quarter rate), communication is continued even under interference influence without decreasing the throughput. The LS-xx allocation is used to decrease the rate. When the total number of subcarriers is 64, Non-Patent Document 2 recommends LS-8 based on the relationship between the PAPR and the error rate characteristics (see Non-Patent Document 2).
Hereinafter, a method in which single-carrier signals are generated by a method of generating multi-carrier signals, such as DFT-s-OFDM or CI, and then the generated spectra are controlled for communication, is collectively called a “spectrum control single carrier communication method” in this description    [Non-Patent Document 1] 3GPP R1-050702, “DFT-Spread OFDM with Pulse Shaping Filter in Frequency Domain in Evolved UTRA Uplink,” NTT DoCoMo    [Non-Patent Document 2] The 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Wireless communications (PIMRC' 06), “MICROSCOPIC SPECTRUM CONTROL TECHNIQUE USING CARRIER INTERFEROMETRY FOR ONE-CELL REUSE SINGLE CARRIER TDMA SYSTEMS,” Osaka University